1. Field of the Invention
The present invention relates to a charge-controlling semiconductor integrated circuit and a charging apparatus for a secondary battery, and especially relates to a charge-controlling IC (semiconductor integrated circuit) equipped with a back-flow preventing function and a charging apparatus equipped with an AC-DC converter to generate an input voltage for the charge-controlling IC.
2. Description of Related Art
A charging apparatus for a secondary battery uses an IC having a charge-controlling circuit to control a charging current with a charge-controlling transistor composed of a MOSFET (insulated gate field effect transistor; hereinafter referred to as MOS transistor) provided between an input terminal to which a direct voltage from an AD adapter is input and an output terminal to which the secondary battery is connected.
In such charge-controlling IC, an input voltage, which is used as a source voltage for an inner circuit of the IC, sometimes decreases during battery charge. When the input voltage becomes below a charging voltage, even if the charge-controlling MOS transistor is turned off, there is a possibility that a reverse current (back-flow) flows to the input terminal side through a parasitic diode existing between a drain and backgate (substratum or well region).
Conventionally, there has been known as techniques for preventing a back flow a technique to provide a back-flow preventing diode between an input terminal and output terminal in series with a current-controlling MOS transistor, and a technique to provide a back-flow preventing transistor and back-flow detecting circuit to prevent a back flow. However, since a back-flow preventing element is provided in series with the current-controlling MOS transistor according, there is a defect in the above techniques that losses in these elements are large.
Consequently, as shown in FIG. 6, there has been a back-flow preventing technique to prevent reverse current from flowing to an input terminal side through a parasitic diode of the Q1 by providing switches SW1, SW2 between a source/drain of the current-controlling MOS transistor Q1 and a substratum (backgate), by providing a comparator CMP to compare an input voltage and an output voltage, by turning off the current-controlling MOS transistor Q1 when the output voltage becomes higher than the input voltage, and by switching/controlling the SW1, SW2 to apply the higher voltage to the backgate of the Q1.
In addition, as an invention to prevent a back flow by switching the voltage to be applied to the backgate of the current-controlling MOS transistor, there is known techniques disclosed in Japanese Patent Application Publication Lain-Open No. 2004-213697 and Japanese Patent Application Publication Lain-Open No. 2004-280704.
By the back-flow preventing technique to switch the backgate voltage as shown in FIG. 6, the following problem may occur by an input offset which spontaneously occurs in the comparator CMP in manufacturing process. When the input offset Vof occurs on a minus side, since the current-controlling MOS transistor Q1 is turned off at the time when an output voltage VBAT becomes higher than a voltage (VDD-V of) which is lower than an input voltage VDD by the input offset Vof as shown in FIG. 7A, there is a possibility that a battery charge is stopped before the secondary battery is full-charged.
On the other hand, when the input offset Vof occurs on a plus side, the current-controlling MOS transistor Q1 is not turned off until the input voltage VDD becomes lower than a voltage (VBAT-Vof) which is lower than the output voltage VBAT by the input offset Vof as shown in FIG. 7B. For this reason, when an AC adaptor comes off during a battery charge for example, the output side voltage transmits though the Q1 of an on-state so that the input side voltage does not become lower than the output side voltage. As a result, an output of the comparator does not invert, namely the Q1 is not turned off.
When circuits in the charge-controlling IC are configured to operate according to the input voltage VDD, there is a possibility that the VDD lowers along with the VBAT, and current is continuously supplied from the secondary battery side to an inner circuit in the IC through the Q1 of the on-state so that the battery discharges electricity.